The present invention relates to an electrochemical etching apparatus, particularly a CMOS-compatible etching apparatus for etching silicon wafers, as well as a method for etching an etching body according to the species defined in the independent claims.
Electrochemical etching apparatuses, for example, for producing porous silicon or for introducing pores on the surface of silicon, are usually composed of a 2-chamber system, between which a silicon wafer to be etched is clamped as a separating wall, and the two chambers being electrically coupled or connected to one another only by the wafer. Furthermore, electrodes, generally made of platinum, are usually placed in both chambers for the current supply. For example, such an etching apparatus is already described fully and in its essential details by Fujiyama et al in the U.S. Pat. No. 5,458,755.
However, in known etching apparatuses, the problem continually occurs that at least the anodically connected electrode is at least slightly corroded and dissolved during operation, so that initially the electrolyte, and via it the wafer to be etched, becomes contaminated by the dissolved electrode material in the course of the etching process. However, in many cases, such contamination, e.g. by platinum in a silicon production, is not acceptable and impairs the etched wafer or the etching body in its electrical or catalytic properties considerably.
Thus, in particular a silicon wafer, on or in which a layer of porous silicon was produced using an electrochemical etching process and which, in so doing, was contaminated with platinum, is unsuitable for use in a CMOS production (CMOS=complementary metal-oxide semiconductor).
Proposals for solving this problem which are based upon a one-sided metallization of the wafer back side and the use of an only one-sided etching device, the back side of the wafer to be etched forming the metal contact and only the front side being in connection with the etching medium or the electrolyte and, via it, with a platinum electrode, are unsuitable because of the requisite back-side metallization to be applied and the necessary sequence steps in processing the wafer (oxidation, layer depositions, etc.), for which this metallization then stands in the way.
Compared to the related art, the electrochemical etching apparatus of the present invention for etching an etching body and the method of the present invention carried out with it having the characterizing features of the independent claims have the advantage that contamination-free electrochemical etching of an etching body is thereby permitted, at least on the surface the etching body having an etching material to be etched or being made of it. This holds true in particular for producing porous silicon from a silicon wafer. Consequently, the etching body is not impaired, particularly in its electrical, i.e. electronic or catalytic properties, by this etching.
Avoidance of contamination and/or impairment, associated at least partially with the contamination, of the electrical or catalytic properties of the etching body obtained after the etching is very advantageously achieved in that the material of the electrodes, which are directly connected to the etching body via electrolytes in the etching apparatus, is in each case selected according to the material of the etching body. When working with the etching method of the present invention using such an etching cell, at least one of these electrodes, at least with its side facing the etching body, is very advantageously used as a sacrificial electrode.
Advantageous further developments of the present invention result from the measures indicated in the dependent claims.
Thus, it is particularly advantageous if the material of a first electrode and/or the material of a second electrode, which are connected as cathode and anode, respectively, and are electrically connected to the etching body via a suitable electrolyte, is the same material as the etching material of the etching body to be etched. In this context, it is sufficient if the etching body, at least on the surface, has the etching material to be etched or is made superficially of it, and if the first and/or second electrode, at least on the surface, has a corresponding electrode material or is made superficially of it.
Furthermore, it is very advantageous if the etching material of the etching body is at least weakly electrically conductive, silicon preferably being used as etching material or a silicon wafer being used as the etching body. It is also advantageous if the first electrode material and the second electrode material of the first and second electrode, respectively, is a CMOS-compatible material, and in particular is not an element selected from the group platinum, gold, iridium, rhodium, palladium, silver or copper. Consequently, the etching apparatus of the present invention is particularly suitable for producing porous silicon on a silicon wafer, contamination of the wafer with silicon-foreign substances such as platinum or palladium being prevented, for example, by the use of silicon electrodes.
In this context, understood by a CMOS-compatible material according to the general usage in semiconductor technology is a material which does not negatively affect the electrical properties of a circuit produced with it.
Accordingly, to be understood by a material contaminating the etching body is, in particular, a CMOS poison or a material which, given its embedment, forms deep imperfections in the etching body, that is to say, imperfections whose energy levels lie in the middle of the gap between conduction band and valence band of the material to be etched and which consequently give rise to a high transition matrix element for the recombination of electrons and holes in the etching body (xe2x80x9crecombination seedxe2x80x9d).
Advantageously suitable as electrode materials for the first and second electrodes, respectively, are especially compounds from the group of the at least weakly conductive compounds of the elements silicon, carbon, nitrogen, oxygen, titanium, aluminum, boron, antimony, tungsten, cobalt, tellurium, germanium, molybdenum, gallium, arsenic and selenium, particularly SiC, SiN, TiN, TiC, MoSi2 und GaAs, as well as pure electrode materials from the elements silicon, titanium, tungsten, molybdenum and carbon, particularly graphite.
Generally, the materials commonly used as contact materials in semiconductor technology are also suitable as electrode materials, since they do not diffuse deeply into the etching body upon striking it, and therefore during the etching, but rather react superficially with the etching body, i.e. are locally bound forming, for example, silicides, and thus remain restricted on the surface of the etching body. Thus, in this sense they do not contaminate the etching body and also do not impair it in its electronic properties, particularly with respect to the use in, or compatibility with, a CMOS-compatible production line.
Advantageously, in each case, the respective electrode material is specifically selected taking into consideration the material of the etching body and the electrolytes used. In addition, the first electrode and/or the second electrode and/or the etching body are advantageously planar, particularly in the form of wafers, the electrodes for use as sacrificial electrodes very advantageously being substantially thicker than the actual etching body besides, so that if desired, they can be recovered, freed from contamination and reused. The exchange cycles of the electrodes are thereby advantageously prolonged.
The electrochemical etching cell is advantageously constructed such that a first chamber and a second chamber are provided which are each filled at least partially with an electrolyte and which are separated spatially from each other via a separating device. Each of the two chambers is electroconductively connected via an electrolyte to an electrode, the etching body at least region-wise being the separating device, and at the same time very advantageously also being the only, at least weakly conductive electrical connection between the two chambers and the electrodes connected as cathode and anode, respectively.
A further very advantageous embodiment of the invention provides that, in addition to the two chambers already mentioned, the electrochemical etching cell is provided with a further third chamber, or a further third chamber and a further fourth chamber, which are each at least partially filled with an electrolyte and in each case are spatially separated from the first chamber and second chamber, respectively, via a further separating device. In this case, the electrolyte in the third and fourth chamber, respectively, is very advantageously electroconductively connected only to the second and first electrode, respectively, which in turn simultaneously serve at least region-wise as the separating device between the third and fourth chamber and the first and second chamber, respectively.
In this connection, it is particularly advantageous if, only with their surface facing the etching body, are the especially planar first and/or the second electrode in contact with the electrolyte that is in contact with the etching body, so that the electrolyte in the third and fourth chamber is prevented from mixing with the electrolyte in the first and second chamber, respectively. Therefore, for simpler electrical contacting of the electrodes, the side of the first and/or second electrode facing away from the electrolyte of the first or second chamber can be provided superficially at least region-wise with a metallization or a doping or, for example, in the case when the electrode is composed of a plurality of layers, can be made of a metal, which combines the advantage of a simple constructional design of the etching cell with the purposeful adaptation of the electrode material to the respective etching material without contacting or contamination problems occurring.
Furthermore, for easy electrical contacting of the first and second electrodes via the respective electrolyte, provision can be made in the third and fourth chamber, respectively, for an additional bath electrode, particularly a platinum or palladium electrode, dipping into an electrolyte located there.
Incidentally, the electrolytes in the individual chambers of the etching apparatus according to the present invention can advantageously also be different from one another, the first and second chambers in which the actual etching of the etching body takes place being advantageously filled with hydrofluoric acid or a mixture of hydrofluoric acid and ethanol, and the third and fourth chambers, for example, being filled with diluted sulfuric acid as contact electrolyte.
Furthermore, the individual chambers are very advantageously capable of being filled separately with electrolyte and emptied separately, thus allowing a problem-free exchange, for example, of a contaminated electrolyte in each chamber at any time. Consequently, in addition a simple exchange of an exhausted or contaminated first and/or second electrode used as a sacrificial electrode is made possible easily and quickly at any time.
Incidentally, the first and/or second electrode is advantageously electrically contacted via the electrolyte, filled into the third and fourth chamber, respectively, to a bath electrode located there, and thus is connected to an external voltage supply which impresses a current on the etching apparatus during operation.
Moreover, the problem-free exchangeability of the sacrificial electrodes, i.e. the first and/or the second electrode, very advantageously makes it possible, in a simple manner, to investigate the suitability of different electrode materials such as graphite, for example, during the etching of an etching body, and in so doing, to optimize the electrode materials to the respective material of the etching body.
In addition, to homogenize the etching of the etching body in the etching apparatus according to the present invention, a tunnel of non-conductive material, particularly polypropylene, can advantageously be provided in a manner known per se.